1. Field of the Invention
The present invention relates to an autofocus mechanism adjusting a focal position with respect to a work piece.
2. Description of Related Art
Conventionally, an autofocus mechanism used in a measurement device is known (see Japanese Patent Laid-open Publication No. H07-105547). The autofocus mechanism uses a method emitting light at a work piece and identifying a focal position based on light reflected by the work piece. Specifically, the reflected light received by a photodiode is converted to an electric signal. Then, a low-strength electric signal is amplified by a high-magnification amplifier, while a high-strength electric signal is amplified by a low-magnification amplifier. Then, the focal position is identified based on the amplified electric signals.
However, offset and gain vary in the high-magnification amplifier and the low-magnification amplifier of the above-noted autofocus mechanism. Therefore, accuracy in identifying the focal position deteriorates. Moreover, feedback control, which tracks a surface of the work piece based on the focal position, cannot be performed stably.